This LRO reader emailed LRO.email@example.com looking for help with a jumpy Series I petrol gauge.
On my 1957 Series I the petrol gauge needle jumps about – especially when the tank is about half full. Is this normal?
B Ormerod, Colne, Lancashire
What you describe is typical of many Series Is, though it’s not how they’re supposed to be. Your problem is the so-called ‘gauge unit for petrol tank’ – the little thing on the top corner of your tank. Take off its cover (four screws); then if you reach inside the tank and get hold of the float arm, you can move the float up and down to simulate more or less petrol.
Looking at the gauge unit, you’ll see a pair of contacts moving forward and back along a coil of wire. The wire is simply resistance – the contacts encounter more or less resistance depending on their position along the wire. That’s how the gauge works. But if the moving contacts don’t touch the wire coil cleanly, the gauge needle jumps.
Don’t clean with emery paper, as I’ve seen suggested elsewhere – just brush with a soft brush, squirt with very light oil, and work the float arm manually for a while. That should clean up the bad-contact area (around the halfway point, as you described). Remember the cover’s screws are placed asymmetrically, and just a nip-up is all that’s needed. A wipe of grease around the edge of the gasket will keep out moisture.
This workshop advice appeared in the December 2015 issue of LRO. Back issues are available to download on digital devices here.